(#14985) Fix hiera lookup for calling_module and calling_class
(#14985) Fix hiera does not lookup calling_class and calling_module
(#14985) Fix test, return downcase values and check for nil scope.source
(#14985) Fix exception when nil is returned from inner scope
(#14985) Ensure hostclass finder is private

The nested definition of the recurse_for_hostclass method caused that
method to be created lazily and as a public method. By pulling it out to
be declared outside the calling method it can be made private and
created only once. It has also been renamed to show the intention
instead of the mechanism (recursion).
(Maint) Require Hiera::Config to avoid warning

When the hiera tests were run alone the puppet_backend_spec.rb issued a
warning:

  /Users/andy/work/puppet/spec/unit/hiera/backend/puppet_backend_spec.rb:25:
  warning: toplevel constant Config referenced by Hiera::Config

The warning was caused by a reference to Hiera::Config where Hiera was
defined, but Hiera::Config was not. This resulted in getting the top
level Config constant and a warning being issued. By requiring
hiera/config this problem is avoided.
(Maint) Use real scope in testing

Using the real scope is completely possible in these tests since it has
no dependencies on the outside world and is essentially a value object.
Therefore it should be used in the tests to make them more
comprehensive.
(#14985) Describe behavior of calling_(class|module)

The original tests for calling_module and calling_class used a mock,
when this is really just a value that can be setup. It also didn't
separate out and describe the behavior of the two pieces in the
variations in which they can come up. Specifically the difference
between a `define` and a `class` for the meaning of calling_class.
(Maint) Clarify scope lookup rules

The lookup rules for variables in the puppet language have always been a
bit hard for me to understand. I knew the rules as

  the current scope => inherited => node => topscope

but the code never seemed to express that clearly. This refactors the
`lookupvar` method to extract out a few pieces to make that clearer.
Specifically the mechanism for walking from a class to the enclosing
node (which is why this gets the name "two-step" by some people) has
been called out as a lookup of the `enclosing_scope`. Calling out the
enclosing scope as a specific lookup allows for getting rid of the
`:origin` part of the options hash, which was to provide a mechanism for
skipping over some scope objects.

This also tries to remove some duplication that was occurring because
there was not a consistent symbol table concept. It turns out that the
existing `Ephemeral` fits that nicely and so it now used as the scope's
symbol table as well as for the ephemeral variables.
(#14985) Preserve casing on looked up values

The initial fix for looked up values downcased everything that was being
returned. This causes problems when the data is not a string that the
previous behavior was to only do this for the calling_class and
calling_module. This commit us returns to that behavior.
(Maint) Extract constant strings as constants
(#14985) Alias module_name as calling_module

The new implementation of calling_module turned out to be the same as
puppet's own module_name. This makes it simply and alias so that we keep
that as the definition of calling_module.
Merge pull request #1497 from zaphod42/issue/master/16688-hiera-extra-vars

(#14985) Enable calling_class and calling_module for defined types